Can someone translate the author's commentary? I was confused as to whether or not Mika's lying or not (I'm not good at figuring out context in Japanese).

It's clear from cultural context that she's lying and is in fact a virgin.

Artist said:
守備力高い姉

守備が高いから貞操の守りもばっちりです　友達とかにえっちな話振られたらわたわたしてると思う

"Elder sister with strong defenses.

Considering her defensive nature she's definitely protected her chastity as well. I imagine she gets panicky when her friends talk about something perverted."